1. Field of the Invention
The present invention relates to a vane pump. More particularity, the present invention is directed to a hydraulic system having a vane pump.
2. Background of the Related Art
A conventional vane pump is disclosed in a Japanese Patent Laid-open No. 4(1992)-339187. There, a cylindrical rotor is rotatably supported within a housing, together with a cam-ring. Only the rotation angle of the cam-ring is prevented and the rotor is permitted to rotate by 360 degrees. A ring-shaped space is formed between an outer surface of the rotor and an inner surface of the cam-ring in the housing. A plurality of vanes, located in slits of the rotor, separate a plurality of chambers in the ring-shaped space. A ring-groove is formed on the housing and communicates with the slits. The vanes are pushed onto the inner surface of the cam-ring by centrifugal force and oil pressure acting on the inner side of the vanes in the slits. The oil pressure acting on the inner side of the vanes in the slits is supplied from a discharge port of the vane pump via the ring-groove.
Such a vane pump may be used to supply oil pressure for a power-steering system for a motor vehicle. In operation of the power-steering system, one of two valves is closed and the discharge pressure of the vane pump becomes high. The high discharge pressure is applied to the inner side of the vanes in the grooves so that the pressure strongly pushes the vanes outwardly. However, since the discharge pressure in the ring groove is high, oil in the ring groove leaks past both sides of the vanes and the rotor into the suction side via the ring-groove. The discharge pressure then becomes low and the pressure at the inner side of the vanes in the grooves also becomes low. Therefore, the vanes are not sufficiently pushed onto the inner surface of the cam-ring and oil in the chambers communicating with the discharge port leaks past the outer ends of the vanes into the other chambers. The pumping capacity of such vane pump is thereby lowered.